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May 2000

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Subject:
From:
David Douthit <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sun, 21 May 2000 12:52:15 -0700
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Stephen

I want to thank you for your "spare time" research activities. They are
saving me a lot of time.

I have read the NASA report  and I want to make a few comments about it.



1. The information contained in this report is very “dated”. The newest
data is from 1992.

2. Most of the conclusions are based on testing of DIP PEM packaging for
TH assemblies.

3. No airborne contamination loading was used for any testing (except
for a limited salt fog test).

The use of this report to justify the acceptability of PEMS for use in
high reliability assemblies is a dangerous “Leap of Faith”. PEMS
currently used for assemblies bare virtually no resemblance to the
components evaluated in this report. The changes in size, materials,
operational voltage levels, frequencies, and signal strengths have a
combined effect of lowering the amount of moisture plus contamination
levels needed to disrupt circuitry operation. This report is only of
limited value for the component types tested.

The lack of airborne contamination loading during testing means they are
only useful for determining
the “as built” cleanliness, purity of materials, and processes used in
the manufacturing of components. It does not provide any useful
information as to the durability of components when exposed to real
world
“harsh environments”. Salt fog testing does not correlate to any real
world loading conditions and is primarily used to detect “infant
mortality” failure modes. With the onset of “performance based
contractual
requirements”, the lack of real world contamination load testing of
circuitry causes very ugly situations to develop.

It is no longer very useful to evaluate PEMS independently of the
substrates/laminates to which they are attached. The changes in the
design and packaging of PEMS are reflected by corresponding changes in
substrates. This will cause the whole assembly to become more sensitive
to moisture/temperature/contamination combinations.

This report is very useful in several respects. It demonstrates the
dangers of transferring legacy test data from obsolete hardware to
current production equipment. It can be very misleading. It also
highlights the time lag issue, i.e. by the time testing protocols are
agreed to, testing is done, and evaluation is completed there is a very
good chance the the component is obsolete!

D. A. Douthit

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